Method of inspecting an operation of sealed closure by welding the end of a filling channel traversing the upper plug of a nuclear fuel rod

ABSTRACT

The invention concerns a control method in an installation ( 9 ) filling a pencil ( 1 ) with inert gas under pressure inserted in the sheath ( 2 ) of the pencil via a filling channel passing through the top cap ( 4 ) of the pencil ( 1 ). The sealed closure of the pencil ( 1 ) after it has been filled is carried out by welding means such as a laser beam melting part of the top cap ( 4 ) of the fuel pencil ( 1 ) at the periphery of the pencil filling channel ( 7 ). Prior to the sealed closure of the pencil ( 1 ) filling channel ( 7 ) by welding, the method consists in scanning the end of the cap whereon the filling channel emerges in a circular opening, to obtain a digitised image and in a determining by analysing the digitised image, the position of the centre of the opening of the circular inlet of the channel filling the cap ( 4 ), relative to a reference position and the diameter of the inlet of the filler channel and deducing therefrom whether it is possible to perform the welding, and in the event the welding is carried out, scanning the end of the cap ( 4 ) after welding and determining the presence of a weld seam providing the top cap ( 4 ) of the fuel pencil ( 1 ) with sealed closure.

FIELD OF THE INVENTION

[0001] The invention relates to a method of inspecting an operation ofsealed closure by welding the end of a filling channel traversing theupper plug of a nuclear fuel rod.

BACKGROUND

[0002] Nuclear reactors such as nuclear reactors cooled by pressurizedwater comprise a core consisting of fuel assemblies in which energy inthe form of heat is produced during operation of the reactor.

[0003] Each of the fuel assemblies generally consists of a bundle ofmutually parallel fuel rods held in the framework of the fuel assembly.Each of the fuel rods comprises a tubular padding made of a materialwhich weakly absorbs neutrons such as a zirconium alloy, in whichnuclear fuel pellets are stacked, for example sintered pellets of UO₂uranium. The tubular cladding is closed at its ends by plugs, each ofwhich comprises a cylindrical part which is engaged coaxially in an endpart of the cladding. The plug and the cladding are then fastened one tothe other by welding around a circular line located in a plane which issubstantially perpendicular to the axis of the cladding and of the plug.

[0004] One of the two plugs for closing the cladding of a fuel rod,which is called the upper plug, because it closes the upper end of therod inside the fuel assembly in the service position of the fuelassembly in the core of the nuclear reactor, is traversed axially by achannel, wherein the cladding of the rod is filled, around the nuclearfuel pellets, by an inert pressurized gas such as helium which protectsthe fuel pellets against oxidation and promotes heat exchange betweenthe pellets and the cladding of the rod when the rod is in service inthe core of the nuclear reactor.

[0005] The manufacture of the fuel rods requires numerous successiveoperations in order to fill the cladding with the fuel pellets, to placeand weld the plugs and to introduce an inert pressurized gas such ashelium into the cladding sealed shut by the plugs, and to close thefilling channel of the plug, after filling. Numerous inspections must becarried out at all steps of the fuel rod manufacture, so as to attainfuel rods which are completely free of defects.

[0006] In particular, sealing the closure by welding the filling channelof the upper plugs must be subject to rigorous inspection.

[0007] The filling with pressurized helium of the cladding of the rodscontaining the fuel pellets and sealed shut by the plugs is carried outin a filling apparatus in which the upper end part of the fuel rod,comprising the upper plug traversed axially by the filling channel, isinserted. The end of the upper plug, on which the filling channelemerges in the form a circular inlet opening extended axially by achamfered part of the filling channel, is placed so as to be able toengage with a valve for closing and opening the chamfered end part ofthe filling channel. In the open position of the valve, the aircontained inside the fuel cladding is evacuated, then the rod is filledby pressurized helium entering inside the cladding via the fillingchannel of the upper plug. Finally, in the filling apparatus, the end ofthe filling channel is sealed shut by a weld obtained by melting acentral part of the plug adjacent to the end part of the fillingchannel. Generally, the material of the plug is melted to carry out theweld under the effect of a laser beam directed axially on the chamferedinlet part of the filling channel. The weld obtained by a pulsed laseris generally formed from three successive spot welds, in order toincrease the safety and the production quality of the closure. Thechamfered inlet part of the filling channel is generally known by theterm “seal weld”.

[0008] The weld is carried out under satisfactory condition only if theaxis of the laser beam for melting the material of the plug, along theperiphery of the chamfered inlet part of the filling channel, iscentered accurately with respect to the circular inlet opening of theseal weld and if this circular opening has a diameter the size of whichis predetermined and defined very accurately.

[0009] Furthermore, after welding, the quality of the spot welds can beinspected, in order to determine whether the fuel rod compliesperfectly.

[0010] In the prior art, the inlet opening of the filling channel orpressurization hole is positioned manually and with simple visualmonitoring, before the welding operation. The mechanical adjustment formanually controlling the positioning of the plug and of thepressurization hole cannot enable the perfect positioning of thepressurization hole to be guaranteed in all cases during welding. Thereare possibilities of maladjustment or blockage of the rod, which resultin poor positioning. In addition, the visual monitoring of thepositioning of the filling channel does not make it possible to obtain ahighly efficient adjustment either.

[0011] The quality of the spot weld is inspected by X-ray scanning. Thisinspection makes it possible to detect porosities in the weld but doesnot allow the position and size of the weld to be verifiedsatisfactorily.

[0012] Furthermore, the inspection must be carried out on a separatestation of the filling apparatus, which makes the rod manufactureoperations more complex and longer.

[0013] Where the sealing weld is carried out by laser welding, thequality of the laser weld depends on the geometry of the chamfer of theinlet part of the filling channel; in particular, it is necessary toverify that the chamfer has an inlet opening whose diameter, whichconstitutes the maximum diameter of the chamfer, complies with extremelystrict size requirements.

SUMMARY

[0014] The aim of the invention is therefore to propose a method ofinspecting an operation of sealed closure by welding an end opening of afilling channel axially traversing an upper plug for closing thecladding of a fuel rod for a nuclear reactor, the cladding of the rodcontaining a plurality of pellets of nuclear fuel stacked in the axialdirection of the cladding and two closure plugs, one of the plugs or theupper plug being traversed axially by the channel for filling thecladding of the rod with an inert gas and the sealed closure by weldingof the filling channel of the upper plug being carried out after fillingthe cladding with inert gas, in a filling apparatus, by melting centralpart of the end of the upper plug adjacent to the opening of the fillingchannel, this method making it possible to inspect the conditions forimplementing and carrying out the sealed closure of the upper plug bywelding, efficiently and without extending the time needed for themanufacture of the fuel rod.

[0015] With this aim, prior to the sealed closure of the fillingchannel, the fuel rod being in the position for filling and for sealedwelding of the upper plug in the filling apparatus, images are acquiredof the end of the plug on which the substantially circular inlet openingof the filling channel emerges, and in order to obtain a digitizedimage, the position of the center of the circular inlet opening of thefilling channel is determined with respect to a reference position andthe diameter of the inlet opening of the filling channel by analyzingthe digitized image. It is thereby deduced whether it is possible toweld the filling channel. Where the sealed closure of the fillingchannel is carried out by welding, images are acquired of the end of theupper plug after welding and the presence and the position of a weld forsealed closure of the filling channel are determined.

DETAILED DESCRIPTION

[0016] In order for the invention to be properly understood, theimplementation of the inspection method according to the invention willnow be described, by way of example, with reference to the appendedfigures, in the case where the filling channel of an upper plug of afuel rod is sealed shut by laser welding.

[0017]FIG. 1 is a view in partial axial section of the upper end of afuel rod, in a manufacturing phase before it is filled with inert gasand the upper plug is sealed shut.

[0018]FIG. 2 is a view in partial axial section similar to the view ofFIG. 1 showing the upper end part of the rod after the upper plug issealed shut by welding.

[0019]FIG. 3 is a schematic view of a station for filling with inert gasand for the sealed closure by laser welding of the upper plug of fuelrods and of a device for inspecting the weld of the upper plugs of thefuel rods.

[0020]FIG. 4 is a explanatory image showing the successive searchoperations carried out before welding in order to implement the methodaccording to the invention.

[0021]FIG. 5 is a graph giving the gray level of points of a column orrow of the image which is obtained during the inspection, before weldingthe filling channel of the upper plug of a fuel rod.

[0022]FIG. 6 is an example of an image appearing on the screen of theinspection device on implementing the method according to the inventionbefore welding the filling channel of the upper plug of a rod.

[0023]FIG. 7 is an example of an image appearing on the screen of theinspection device on implementing the method according to the inventionbefore welding the filling channel of the upper plug of a rod.

[0024]FIG. 8 is an example of an image appearing on the screen of theinspection device on implementing the method according to the inventionbefore welding the filling channel of the upper plug of a rod.

[0025]FIG. 9 is an explanatory image illustrating the successive searchoperations carried out on implementing the method according to theinvention, after welding the filling channel of the upper plug of a fuelrod.

[0026]FIG. 10 is a graph illustrating the gray level along a row or acolumn of the image supplied by the inspection device, after welding thefilling channel of the upper plug of a rod.

[0027]FIG. 11 is an example of an image appearing on the screen of theinspection device, as a result of inspecting the weld of the fillingchannel of an upper plug of a fuel rod.

[0028]FIG. 12 is an example of an image appearing on the screen of theinspection device, as a result of inspecting the weld of the fillingchannel of an upper plug of a fuel rod.

[0029]FIG. 13 is an example of an image appearing on the screen of theinspection device, as a result of inspecting the weld of the fillingchannel of an upper plug of a fuel rod.

DETAILED DESCRIPTION

[0030] The upper end part of a fuel rod for a pressurized-water nuclearreactor, denoted generally by the reference 1, is illustrated in FIG. 1and in FIG. 2.

[0031] The rod 1 comprises, in particular, tubular cladding 2 made ofzirconium alloy which contain fuel pellets 3 and which is closed at itsupper end, represented in FIG. 1 by a plug 4 constituting the upper plugof the fuel rod.

[0032] The plug 4 is generally made up of zirconium alloy and comprisesa part 4 a which is engaged, virtually without clearance, in the endpart of the bore 2. After filling the cladding 2 with fuel pellets 3,the closure plugs of the cladding, such as the upper plug 4, are engagedin the end parts of the cladding, the sealed attachment of the plugbeing provided by a weld line such as 5 made by melting the material ofthe plug and of the cladding, along a circular line.

[0033] A spring for holding the fuel pellets is inserted between theupper plug 4 and the end of the column of fuel pellets 3.

[0034] The upper plug 4, which is axisymmetric about an axis 6,comprises an end part 4 b opposite, in the direction of the axis 6, toits end 4 a engaged in a coaxial arrangement inside the cladding 2. Theoutside of the end part 4 b of the plug 4 has, successively in the axialdirection 5, from the upper end of the plug, a frustoconical plug, ashoulder and a cylindrical part along which the diameter of the plug isat a minimum. The minimum-diameter part of the plug and the shouldermake it possible to engage and to fasten the jaws of a gripper tool,during fuel rod maintenance operations.

[0035] The plug 4 is traversed, in the direction of the axis 6, alongits whole length, by a channel 7 which will be denoted subsequently as afilling channel, this channel 7 making it possible to fill the claddingof the rod with an inert pressurized gas, as will be explained below.The filling channel 7 of the plug 4 comprises several successive parts,the diameters of which generally decrease from the end 4 a of the plugengaged in the cladding 2 to the outer end part 4 b of the plug.

[0036] At an upper end or outer end, the channel 7 comprises a terminalpart 7 a of frustoconical shape or a seal weld flared from the bottomupwards, i.e. from the inside to the outside of the plug, which emergeson the upper end surface 8 of the plug along a circular inlet opening,the center of which is on the axis 6 common to the plug and the cladding2.

[0037] The seal weld 7 a of the channel 7, along which the channel 7 isclosed by a weld, provides the junction between a part 7 b of thechannel 7 of minimum cross section and the circular inlet opening of thechannel 7 on the upper end surface 8 of the plug 4.

[0038] The small-diameter part 7 b may have a diameter of about 0.6_mmand the circular inlet opening of the terminal frustoconical part 7 a adiameter of about 1.3_mm.

[0039] As illustrated in FIG. 2, after having filled the cladding of therod with an inert pressurized gas, such as helium, the channel 7 issealed shut in its upper terminal part, by a weld 10 obtained by meltingthe material of the plug in the central region of its upper end part, atthe periphery of the seal weld 7 a and consisting, for example, of threelaser spot welds.

[0040] In FIG. 3, the upper plug 4 of the fuel rod 2 is filled andsealed in a filling and welding apparatus 9.

[0041] The filling and welding apparatus mainly comprises a enclosure,one wall of which comprises an opening for the passage of a fuel rod 2in a horizontal arrangement, the upper end of the rod being insertedinto the enclosure, so that the upper plug 4 of the fuel rod 2 engageswith a valve 11, the closure member of which makes it possible to closeor to open the terminal end of the filling channel of the plug.

[0042] The enclosure of the filling and welding apparatus 9 comprises apart located opposite the part for introducing the rod 2, on the wallsof which are mounted a laser beam welding device 12 and an opticalassembly 13 making it possible to position the laser beam in order toweld the sealed closure of the filling channel of the upper plug 4 ofthe fuel rod 2 and which is also used to implement the inspection methodby analyzing the optical image according to the invention.

[0043] A mirror 14 for reflecting and focusing the laser welding beamemitted by the laser welding device 12 is placed inside the second partof the enclosure 9. The device 12 comprises an optical fiber 15connected to a laser source and a collimator 16, so as to send, by thereflection and focusing mirror 14, a laser beam of axial direction, onthe end part of the filling channel 7 constituting the seal weld.

[0044] In order to carry out the sealed closure of the filling channelby a weld 10 under satisfactory conditions, the laser beam may bedirected accurately onto the center of the circular inlet opening of thefilling channel 7 of the upper plug 4. In order to adjust the positionof the laser welding beam, the optical illumination and adjustmentassembly 13, which comprises an illumination device 17 and an opticalsighting assembly comprising a reticule, is used so as to identify thedirection along which the laser shot is sent.

[0045] In the case of methods of adjusting the welding according to theprior art, adjusting the position of the upper plug of the rod iscarried out manually and verified visually.

[0046] In the case of the method according to the invention, theillumination device 17 for sighting and adjusting the laser shot is usedin order to inspect, on digitized images, the position of the center ofthe inlet opening of the filling channel with respect to the laser beamand the compliance of the inlet opening, in order to determine whetherthe welding of the filling channel, after the rod is filled withpressurized helium, can be carried out satisfactorily.

[0047] The weld, consisting of spot welds, is also inspected by analysisof digital images.

[0048] For this purpose, a digital camera 18 a connected to amicrocomputer 20 comprising a screen for displaying images 19 iscombined with an optic 18 directed in the axis of the enclosure 9 alongwhich the fuel rod 2 is engaged. The microcomputer 20 comprises a videoacquisition card and a digital input/output card enabling themicrocomputer 20 to communicate with a controller managing the laserwelding device 12.

[0049] The microcomputer 20 receives inspection orders from thecontroller, via the digital input/output card, and verdicts establishedfrom the results of the inspection are sent by the microcomputer 20 tothe controller for controlling the laser welding device 12. Informationrelating to the rod and the welding conditions is transmitted to themicrocomputer 20 by the controller of the laser welding device 12.

[0050] A first step of the method for inspecting the sealed closure bywelding the filling channel of the upper plug of a rod consists indetermining the position and the size, that is to say the diameter ofthe inlet opening 8 a of the filling channel 7, consisting of thecircular outline along which the filling channel 7 opens on the endsurface 8 of the upper plug 4 of the rod. This inlet opening constitutesthe large diameter end of the chamfer of the terminal part 7 a of thefilling channel.

[0051] The inspection is carried out on the filling and weldingapparatus which has just been described, during a phase in the course ofwhich the fuel rod is filled with pressurized helium, the valve 11 beingopen.

[0052] An image is acquired of the end surface 8 of the plug and of theinlet opening 8 a of the terminal part 7 a of the filling channel usingthe optic 18 and the digital camera 18 a, the optical image obtainedbeing digitized by the digital camera 18 a and transmitted in digitalform to the inspection microcomputer 20.

[0053] The image obtained may be made visible on the screen 19 of themicrocomputer 20, as shown, for example, in FIGS. 6, 7 and 8.

[0054] The search operations carried out on the circular opening of theseal weld will be explained with respect to FIG. 4.

[0055] As a result of illuminating the end of the upper plug of the rodin an axial direction, the end surface 8 of the rod appears as a lightregion and the terminal part 7 a of the filling channel 7 as a darkregion, the light region and the dark region being separated one fromthe other by a substantially circular line 8 a constituting the inletend of the seal weld.

[0056] Analysis of a digital image of the end part of the upper plugallows determination of the position of the center of the circularopening 8 a with respect to the center of a parameterizable sightingmarker, marking the position of the laser beam with which the closure issealed by welding the filling channel 7. Ideally, the position of thesighting marker and the position of the center of the inlet opening ofthe plug should be coincident.

[0057] The sighting marker 21 comprises a vertical axis and a horizontalaxis which define the center of the sighting marker. It is positionedvisually when the laser welding station is correctly adjusted.

[0058] The edges of the opening 8 a are sought on the image along thehorizontal and vertical axes of the reference sighting marker 21.

[0059] Firstly, the edges of the hole 8 a are sought along thehorizontal axis of the sighting marker 21 or first axis. A number N ischosen, which corresponds to the number of rows of the image which willbe used on each side of the reference 21 in order to define a mean rowalong which the edges of the opening 8 a will be sought.

[0060]FIG. 5 illustrates the variations of the gray levels between theblack and the white, along the mean row determined above. The distanceson the X-axis, i.e. along the row of the image, are expressed in imagepoints or pixels.

[0061] A parameterized threshold value is chosen.

[0062] The threshold value is illustrated by the straight line 22 ofFIG. 5.

[0063] The right edge of the image then the left edge are determinedalong the mean row and by comparing the pixels to the threshold value.

[0064] From the right edge and from the left edge defined on the graph,the position of the center of the hole of the seal weld is determined,for example with respect to an edge of the image as shown by the segment23 and the diameter of the seal weld as shown by the segment 24. Theedges of the hole of the seal weld are then sought along a second axisconsisting of the vertical axis 25 (see FIG. 4) passing through thecenter previously defined or the first center.

[0065] The edges of the inlet opening of the seal weld are sought in thesame way as above, but using the columns of the image instead of therows. In this way, a second center of the circular inlet opening and thediameter along the vertical axis are defined.

[0066] The edges of the circular inlet opening 8 a of the seal weld arethen sought, along the horizontal axis 26 or third axis passing throughthe second center defined by the search on the vertical axis. Theposition of the center of the circular opening 8 a and the diameter ofthe circular opening along the horizontal axis are deduced therefrom,this third determination of the center of the opening supplying theactual center taken into account.

[0067] It is checked that the values obtained for the diameter along thevertical axis 25 and along the horizontal axis 26 are consistent, thatis to say that they do not differ by a value greater than a chosenthreshold value.

[0068] The pixels are then converted into millimeters and the diameterof the circular opening 8 a is compared with threshold values defined bythe “minimum diameter and maximum diameter” parameters of the opening 8a.

[0069] The position of the center of the circular opening 8 a defined byits distance to the center of the sighting marker 21 is compared with athreshold value corresponding to a “tolerance of center position”parameter. The results are displayed on the screen of the microcomputer20, as shown in FIGS. 6, 7 and 8. In each case, a first value isdisplayed in millimeters corresponding to the position of the center ofthe circular opening 8 a, a second value is displayed in millimeterscorresponding to the diameter of the circular opening 8 a and a verdictindicating the compliance or the noncompliance of the measured values isdisplayed. Where the measured values comply, an execution command istransmitted to the controller of the laser beam welding device 12.Otherwise, a blocking command is transmitted.

[0070] In the case of FIG. 6, the values identified comply, the distancebetween the center of the opening 8 a and the center of the sightingmarker being less than a chosen threshold value and the diametermeasured being between the acceptable minimum diameter and theacceptable maximum diameter.

[0071] In FIG. 7, the opening 8 a is offset with respect to the centerof the sighting marker 21, such that the distance between the center ofthe opening 8 a and the center of the sighting marker is greater thanthe chosen threshold value. A verdict of noncompliance is thereforetransmitted. On the other hand, the diameter in this case is between themaximum and minimum threshold values.

[0072] In FIG. 8, the diameter measured on the image of the opening 8 ais less than the minimum threshold value. A verdict of noncompliance istransmitted. In addition, the position of the center of the opening 8 ais slightly offset with respect to the position of the sighting marker,the distance between the center of the opening 8 a and the center of thesighting marker is however less than the threshold value.

[0073] A fault verdict is also transmitted when it has not been possibleto mark the edges of the opening 8 a in the course of the searches asdescribed above.

[0074] In the case of a verdict of compliance, a welding executioncommand is sent to the controller of the laser welding device 12. Thewelding is carried out by the pulsed laser which melts the metal of theupper plug in a peripheral region of the terminal part 7 a or seal weldof the filling channel 7. A spot weld is produced closing off the inletof the small-diameter part 7 b of the filling channel 7, then twosuccessive pulses are produced to form the weld 10.

[0075] On its upper surface, the weld 10 is in the shape of a dish orcrater due to the flow and to the deposition of the metal in the hollowcentral part of the plug.

[0076] After welding, since the fuel weld is in position in the fillingand welding apparatus 19, it is possible to inspect for the present andcharacteristics of the spot welds.

[0077] For this purpose, the end surface of the rod 8 is illuminated,using the illumination apparatus 17, thereby sending light in an axialdirection onto the surface of the rod, and images are acquired of theend surface 8 of the rod using the optic 8 and the digital camera 8 a.The digitized image is transmitted to the microcomputer_20.

[0078] The image of the end part of the plug can be displayed on thescreen, as shown, for example, in FIGS. 11, 12 and 13.

[0079] The search operations carried out on the spot weld will beexplained with respect to FIG. 9.

[0080] On the image, the end surface 8 of the plug and the reflection 27produced by the light reflected by the crater of the central part of thespot weld 10 appear as the light part.

[0081] By analyzing the digitized image, the distance from the center ofthe reflection 27 to the center of the parameterizable sighting marker21, which is shown on the screen, is determined.

[0082] The position of the center of the reflection 27, that is to saythe distance from this center to the center of the sighting marker, iscompared with a “tolerance of center position” parameter and thediameter of the reflection is compared with threshold values defined by“minimum and maximum diameter” parameters.

[0083] The horizontal mean rows are constructed around the reference,then a maximum number of pixels greater than the threshold, and which bepositioned in the reflection, are sought can.

[0084] The edges of the reflection 27 are then sought along a first axisconsisting of the horizontal axis passing through the position foundabove. For this purpose, a mean row is defined, in the way indicatedabove and a graph is drawn representing the variation of the gray levelsalong the mean row, as shown in FIG. 10.

[0085] A threshold value, represented by the horizontal straight line 28in FIG. 10, is defined.

[0086] The right edge and the left edge of the reflection are sought bycomparing the successive pixels along the mean row with the thresholdvalue. A first position of the center of the reflection is calculatedwith respect to an edge of the image, this position being represented bythe segment 29 in FIG. 10.

[0087] A first diameter of the reflection between the right and leftedges is also calculated, this diameter being represented by the segment30 in FIG. 10.

[0088] The edges of the reflection along a second axis consisting of avertical line passing through the first center defined above aredetermined by carrying out a search identical to the previous one but onthe columns of the image.

[0089] The consistency of the values obtained are verified for thediameter of the reflection 27 along the first and second axes.

[0090] The pixels are converted into millimeters and the diameter iscompared with the “minimum and maximum diameter” threshold values andthe position, with the “tolerance of center position” parameter.

[0091] The results are displayed on the screen of the microcomputer, asshown in FIGS. 11, 12 and 13.

[0092] In each case, one of the items of information displayed indigital form relates to the position and the other to the diameter ofthe reflection. The position of the reflection corresponds to thedistance in millimeters between the center of the reflection and thecenter of the parameterizable sighting marker forming the reference. Theposition of the center of the reflection corresponds to the position ofthe center of the spot weld.

[0093] In the case of FIG. 11, both the position and the diameter of thereflection are satisfactory. A verdict of compliance is then displayed.

[0094] In the case of FIG. 12, the center of the reflection 27 is offsetwith respect to the center of the reference 21 by a value greater thanthe predetermined threshold value.

[0095] A verdict of noncompliance is therefore transmitted. However, thediameter of the reflection is satisfactory, that is to say between theminimum admissible value and the maximum admissible value.

[0096] In the case of FIG. 13 there is no reflection 27, which resultsin the absence of the spot weld in the seal weld of the filling channel7 of the upper plug of the rod. A verdict of noncompliance istransmitted.

[0097] In the case where a verdict of compliance is transmitted, thefuel rod may be considered as correctly welded and may be accepted atthe end of manufacturing.

[0098] The method according to the invention also makes it possible todetermine the depth over which the weld has been carried out and, inparticular, when only the first spot weld has been carried out, a verysmall reflection coming from the bottom of the seal weld is observed. Inthis case, the absence of the second and third spot welds is easilydetected from the measurement of the diameter of the reflection.

[0099] Examination of the graph giving the gray levels along a referenceline passing through the reflection of the spot weld also makes itpossible to determine the position of the center of the dish of the spotweld which is denoted by the reference 31 in FIG. 10.

[0100] Analysis of the optical images produced by the microcomputer 20is carried out using software.

[0101] The invention makes it possible, in the apparatus for filling andsealing the rod, to inspect the diameter of the opening of the seal weldof the upper plug of the rod and its positioning with respect to thelaser welding beam, so as to determine whether or not it is possible tocarry out the sealed closure by welding the rod filled with pressurizedhelium.

[0102] Secondly, the invention also makes it possible to verify that thespot weld has been carried out in a compliant manner. All the operationsare carried out in parallel on the filling and welding apparatus.

[0103] The method according to the invention makes it possible to avoidany operation of maintaining the rods between the filling and sealedclosure apparatus and an inspection station and the verdict relating tothe compliance of the spot weld is available from the end of the weldingoperation.

[0104] Information relating to the operation as a whole (positioning,welding and inspection) may be saved on a hard disk and can be exploitedsubsequently in the form of databases.

[0105] Finally, the illumination used to acquire images is a standardillumination which can be obtained by market available items.

[0106] The invention is not limited strictly to the embodiments whichhave been described.

[0107] It is possible to process the digitized images of the end surfaceof the upper plug before and after welding, by methods other than thosewhich have been described, for example for determining the edges of theinlet opening of the filling channel and of the reflection of the spotweld.

[0108] Finally, the method according to the invention is applicable toany nuclear fuel rod comprising an upper plug traversed by a channel forfilling the rod with an inert pressurized gas.

1. A method of inspecting an operation of sealed closure by welding anend opening (8 a, 7 a) of a filling channel (7) axially traversing anupper plug (4) for closing the cladding (2) of a fuel rod (1) for anuclear reactor, the cladding (2) of the rod (1) containing a pluralityof pellets (3) of nuclear fuel stacked in the axial direction of thecladding (2) and two closure plugs, one of the plugs or the upper plug(4) being traversed by the channel (7) for filling the cladding (2) ofthe rod (1) with an inert pressurized gas and the sealed closure bywelding of the filling channel (7) of the upper plug (4) being carriedout after filling the cladding (2) with the inert pressurized gas, in afilling apparatus (9), by melting an end central part (7 a) of the plug(4) adjacent to the opening (8 a) of the filling channel, characterizedin that, prior to the sealed closure of the filling channel (7), the rod(1) being in the position for filling and for sealed welding of theupper plug (4) in the filling apparatus (9), images are acquired of theend surface (8) of the plug (4) on which the filling channel (7) emergesby means of the substantially circular inlet opening (8 a), in order toobtain a digitized image, the position of the center of the circularinlet opening (8 a) of the filling channel (7) is determined withrespect to a reference position (21) and the diameter of the inletopening (8 a) of the filling channel (7) by analyzing the digitizedimage, it is thereby deduced whether it is possible to weld the fillingchannel (7), where the sealed closure of the filling channel (7) iscarried out by welding, images are acquired of the end of the plug (4)after welding and the presence and the position of a spot weld (10) forsealed closure of the filling channel (7) are determined.
 2. The methodas claimed in claim_1, characterized in that: the reference position(21) consists of the center of a reticule comprising a horizontal axisand a vertical axis, the position of the center of the reticule (21)corresponding to a position for adjusting a welding means (12) such as alaser beam for carrying out the sealed closure by welding of the fillingchannel (7) of the plug (4), in that the edges of the inlet opening (8a) of the filling channel (7) on the end surface (8) of the plug (4) aresought along a first axis of the reticule (21), in that a first positionof the center of the circular inlet opening (8 a) of the filling channel(7) and a first value of the diameter of the inlet opening (8 a) arededuced, the edges of the opening are sought along a second axisperpendicular to the first axis of the reticule passing through thecenter defined above, in that a second position of the center of thecircular opening (8 a) of the filling channel (7) and a second value ofthe diameter of the circular inlet opening (8 a) of the filling channel(7) are deduced, in that the edges of the circular opening (8 a) aresought along a third axis perpendicular to the second axis passingthrough the second center defined above, in that a third position of thecenter of the circular opening (8 a) and a third value of the diameterof the circular opening (8 a) are deduced, the third center of thecircular opening (8 a) being considered as the actual center of theopening (8 a), in that the second and third diameters are compared onewith the other in order to deduce thereby the consistency of the secondand third parameter values considered as parameters of the circularopening (8 a), in that the distance between the third determinedposition of the center of the opening (8 a) and the center of thereticule (21) is determined, and in that the distance calculated betweenthe centers of the circular opening (8 a) and of the reticule (21)together with the calculated value of the diameter of the circular inletopening (8 a) of the filling channel (7) are compared to thresholdvalues in order to determine the compliance of the inlet opening (8 a)of the filling channel and of its positioning and the possibility ofcarrying out sealed closure by welding.
 3. The method as claimed inclaim_2, characterized in that the edges of the inlet opening (8 a) ofthe filling channel (7) are sought along each of the axes, from a graphgiving the gray levels along a mean row or column of the digitizedimage, namely the mean gray levels along N rows or columns parallel tothe axis along which the search is carried out and which are located oneach side of this axis.
 4. The method as claimed in claim_3,characterized in that the edges of the circular inlet opening (8 a) ofthe filling channel (7) are determined by using a threshold value of thegray levels constituting a mean between the gray levels of the image ofthe filling channel and the gray levels of the surface (8) of the upperplug (4) around the circular inlet opening (8 a) of the filling channel.5. The method as claimed in claim_1, characterized in that aftercarrying out the sealed closure of the filling channel (7) of the upperplug (4), for example by means of a laser spot weld (10), a reflection(27) of the spot weld (10) having a central part in the shape of acrater reflecting light directed axially is sought on the digitizedimage of the end surface (8) of the plug, and the position of the centerand the size of the reflection (27) are determined.
 6. The method asclaimed in claim_5, characterized in that the position of a center ofthe reflection (27) is determined with respect to a center of a reticule(21) defined by two horizontal and vertical axes, respectively, on thedigitized image, corresponding to a welding position and the diameter ofthe reflection (27), and in that the distance from the center of thereflection (27) to the center of the reticule (21) and the calculateddiameter of the reflection (27) are compared to threshold values inorder to define whether the upper plug (4) of the fuel rod (1) iscompliant after welding.
 7. The method as claimed in claim_6,characterized in that the edges of the reflection (27) are sought alonga first axis of the reticule (21), in that a first position of thecenter and a first value of the diameter of the reflection (27) arededuced, in that the edges of the reflection (27) are sought along asecond axis perpendicular to the first axis passing through the firstcenter of the reflection (27) defined above, in that a second positionof the center and a second value of the diameter of the reflection (27)are deduced, in that the first and second diameters are compared inorder to verify the consistency of the values obtained, in that thedistance between second center of the reflection (27) and the center ofthe reticule (21) is determined, and in that the distance between thecenter of the reflection (27) and the center or the reticule (21) andthe calculated diameter of the reflection (27) are compared to thresholdvalues in order to determined the compliance of carrying out the sealedclosure by welding the filling channel of the upper plug (4) of the rod(1).
 8. The method as claimed in claim_7, charaterized in that the edgesof the reflection (27) are sought along each of the first and secondaxes, by determining a graph giving the gray levels along a mean rowparallel to the first and second axes respectively, corresponding to amean of the gray levels along several rows or columns of the digitizedimage which are parallel to the first or second axes, respectively, andplaced on each side of the first or second axes.